Non-Contact Assessment of Cardiovascular Function using a Multi-Camera Array

ABSTRACT

A system and method are provided for non-contact cardiac assessment of a subject. Images of the subject are captured from at least two synchronized independent imaging devices spaced equidistant from the subject and positioned such that each imaging device captures an image of the subject different from other imaging devices. Spectral components of the captured images are extracted. The extracted spectral components are analyzed. A signal corresponding to cardiac information is identified in the extracted spectral components. The identified signal corresponding to the cardiac information is extracted from the extracted spectral components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication Ser. No. 62/059,870, entitled “Multi-Camera Array forNoncontact Assessment of Cardiovascular Function,” filed on Oct. 4,2014, the entirety of which is incorporated by reference herein.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to non-contact sensing ofphysiological functions and, more particularly, non-contact sensing ofcardiovascular functions.

2. Description of the Related Art

Cardiac monitoring has been sufficiently demonstrated as a method forassessing wellness, performance, cognitive, and stress states ineveryday, clinical, and mission environments. This monitoring istraditionally derived from one of two different modalities:electrocardiography (ECG) or photoplethysmography (PPG).

Electrocardiography measures electrical potentials produced by thedepolarization and repolarization of the various muscles of the heart asthey pump blood to the body. These electrical potentials propagate tothe skin's surface where they may be measured using surface electrodes.Measures of cardiac activity derived from ECG have been demonstrated, innumerous studies, to be sensitive to physiological changes related tothe demands of piloting aircraft. Cardiac measures of both heart rateand heart rate variability have also been shown to be sensitive toflight maneuvers and segments in an operational tactical airliftaircraft. These contemporary results were utilized to aid in a decisionmaking process related to changes made to the cockpit systems in theaforementioned platforms. Additional studies focused on workloadevaluation in current operational missions, a large portion of which aresimilar to the approach used in the tactical airlift aircraft study asrelated to the assessment of cardiac activity for objective measurementof stress and workload. ECG is often considered the “gold standard” formeasuring cardiac activity in both clinical and research areas becauseit directly measures the aggregate electrical activity produced by themuscles pumping blood in the heart. Unfortunately, ECG requireselectrodes to be affixed to the skin surface, costing time andresources, as well as a potential discomfort to the subject. It also maynot be suitable for long term/repeated monitoring or with individualswith high skin sensitivities. Additionally, ECG may be susceptible toartifact under conditions of motion, which shake the ECG leads ordisturb the electrode-skin interface. Moreover, these problems are alsopresent in more modern “wearable” sensors that are popular in fitness,mobile health and wellness, and consumer electronics arenas.

Another monitoring methodology for observing cardiovascular performanceis plethysmography. Plethysmography is a measurement of blood flow, ormore specifically blood volume, in the peripheral vasculature. Contactphotoplethysmography (PPG) is most common and utilizes (non-specific)light-based sources as a transduction medium. PPG takes advantage ofwell-known absorbent and reflective properties of tissue and blood asthey vary with wavelength(s) of the transducer. The most commonapplication of PPG is in pulse oximetry devices, which are usuallysecured to a finger, toe, or earlobe. Such devices measure arterialoxygen saturation and may also be capable of measuring pulse rate andpulse rate variability (which are often employed as surrogates of heartrate and heart rate variability). Unfortunately, traditional PPG sensorsrequire a mounted sensor which may obstruct physical activity or may beuncomfortable for prolonged monitoring. Furthermore, contact PPG sensorsexperience a high degree of motion sensitivity which may corrupt thecollected signal when the sensor-skin interface is disturbed.

PPG and, subsequently, pulse oximetry, are made possible due to aphenomenon known as an optical window where skin tissue exhibitsrelatively low absorption for visible and near-infrared lightwavelengths between 400-2000 (nm). This property is primarily determinedby the absorption spectra for water and skin pigments, particularlymelanin. By contrast, blood and hemoglobin exhibit significantly higherabsorption for wavelengths in the same region. As light enters thetissue, multiple complex interactions occur including absorption,reflection, scattering, transmission, and fluorescence, althoughabsorptiontends to be more important in the creation of an observed PPGsignal. The total light absorption is a summation of a periodic ACsignal associated with pulsatile blood volume changes and alow-frequency (to DC) offset component associated with absorption by thetissue, venous, and arterial blood. This AC signal component is createdas heart contractions send pulsatile blood volumes to the peripheralvasculature that are observed as small, periodic changes in lightabsorption resulting from the varying blood volume in the underlyingvasculature. The PPG signal may be measured in transmission mode, withthe light source on the opposite side of the tissue as the sensor, or inreflection mode, where the light source and sensor are located on thesame side of the tissue. Reflection mode is most common for imagingphotoplethysmography (iPPG) applications where the photoplethysmogram isderived as a net intensity, over a region of interest, observed fromreflected light that is captured by an imager.

Early work in the area of iPPG demonstrated pulsatile components withfrequency characteristics (and their harmonics) similar to those of thephotoplethysmogram (PPG) could be extracted from monochromatic imageryof a fingertip. This phenomenon was also observed in narrow-bandwavelength imagery of a forearm at wavelengths of 660, 810, and 940(nm), along with a lower frequency component related to respirationrate.

While monochromatic images composed of visible and NIR light ornarrow-band wavelengths in the red and NIR range, undoubtedly inspiredby traditional PPG and pulse oximetry methods were traditionally used,it was demonstrated that superficial arterial pathways in the neck(e.g., the carotid arteries) could be isolated in mid-wave infrared(MWIR) imagery, which also showed spectral peaks in a frequency-domainrepresentation of the MWIR imagery data that were likely to be relatedto pulse rate. Near-infrared region bands were used despite very welldocumented evidence that oxyhemoglobin exhibits the most absorption forwavelengths in the green band, which is largely responsible for thepulsatile component of the PPG that links to cardiac cycle rhythms.

iPPG from facial imagery showed improved iPPG data quality by using red,green, and blue (RGB) channels from visible spectrum imagery as thechannel input space to an independent component analysis (ICA)decomposition. This decomposition improved estimates of cardiac measures(as compared to fingertip PPG) beyond those achieved using only thegreen source channel (where the iPPG signal appears to be mostprominent). By extracting inter-beat interval (IBI) time series from theiPPG component, strong correlations for features related to heart ratevariability were shown as compared to fingertip PPG. Additional workfocused on exploring practical issues of the methodology such as imagerquality, frame rate requirements, facial ROI selection for improvedchannel space signal-to-noise ratio (SNR), and applied testing of iPPGin clinical environments.

However, one practical issue that has not yet been addressed is that ofhead motion artifact and its effect on pulse rate component recoveryfrom the ICA (or other blind source separation approach) decompositionof the imager channel space. Contemporary methodologies either restricthead motion to a limited range or eliminate it completely from thedesign. With cardiac activity monitoring likely playing an importantrole in short-term operational evaluations and decision makingprocesses, as well as long-term research in the areas of appliedneuroscience and closed-loop decision aid systems, the potential useracceptance of a non-contact assessment methodology that is as good as,or better than, traditional contact methods in terms of data quality androbustness would be an invaluable step toward transitioning long-term,persistent physiological monitoring to day-to-day operations.Accordingly, there is a need in the art for improved PPG methodologiesthat are not as susceptible to artifacts from head or other movement ofthe subject.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a method for non-contact cardiacassessment of a subject. Images of the subject are captured from atleast two synchronized independent imaging devices. The imaging devicesare spaced equidistant from the subject and are positioned such thateach imaging device captures an image of the subject different fromother imaging devices. Spectral components of the captured images areextracted. The extracted spectral components are analyzed. A signalcorresponding to cardiac information is identified in the extractedspectral components. Finally, the signal corresponding to the cardiacinformation is extracted from the extracted spectral components. In someembodiments of the invention, capturing the images of the subject fromat least two synchronized independent imaging devices includes capturinghead and neck images of the subject from the at least two synchronizedindependent imaging devices.

In some embodiments, extracting spectral components of the capturedimages includes extracting Red, Green, and Blue components of thecaptured images. In some embodiments, analyzing the extracted spectralcomponents includes performing an independent component analysis on theextracted spectral components. In other embodiments, identifying asignal in the extracted spectral components corresponding to cardiacinformation includes bandpass filtering the extracted spectralcomponents between about 0.3 Hz and about 6 Hz. The extracted spectralcomponents are upsampled to about 1200 Hz using cubic splineinterpolation. A signal is chosen from the extracted spectral componentsbased on an estimate of signal-to-noise ratio (SNR) in an expectedfrequency range of a non-contact PPG (ncPPG) component signal. In theseembodiments, the expected frequency range of a ncPPG component signalmay be about 0.75 Hz to about 4 Hz. In other embodiments, extracting thesignal in the extracted spectral components corresponding to the cardiacinformation includes transforming band power estimate to relative powerby dividing by component signal power outside a peak frequency range butwithin about 0.75 Hz top about 4 Hz. In these embodiments, anindependent component is extracted with a highest relative power.Additionally in these embodiments, the extracted component may besegmented into five non-overlapping windows. A frequency may be foundwith a highest power density in a probable pulse rate range. Thisfrequency may then be converted to beats per minute by 60·f_(max), wheref_(max) is the frequency with the highest power density.

Embodiments of the invention also provide a system for non-contactcardiac assessment of a subject. The system includes at least twoimaging devices configured to individually capture images of thesubject. The imaging devices are spaced equidistant from the subject andpositioned such that each imaging device captures an image of thesubject different from other imaging devices. A data acquisition card isin electrical communication with the at least two imaging devices. Atleast one computer is in electrical communication with the at least twoimaging devices and configured to receive the captured images from theat least two imaging devices, extract spectral components of thecaptured images, analyze the extracted spectral components, identify asignal in the extracted spectral components corresponding to cardiacinformation, and extract the signal in the extracted spectral componentscorresponding to the cardiac information. Trigger signals generated bythe data acquisition card cause images to be captured simultaneously bythe imaging devices and delivered to the computer.

In some embodiments, the subject is positioned at a distance of about 68inches from the imaging devices. Alternate configurations of the imagingdevices may include three imaging devices positioned about 20 degreesapart, spanning about 40 degrees of visibility of the subject, fiveimaging devices positioned about 30 degrees apart, spanning about 120degrees of visibility of the subject, nine imaging devices positionedabout 20 degrees apart, spanning about 160 degrees of visibility of thesubject, four imaging devices positioned about 40 degrees apart,spanning about 120 degrees of visibility of the subject, among others.In some embodiments, the imaging devices are positioned at a height tocapture head and neck images of the subject. The arrangement of theimaging devices in some embodiments are positioned along a semi-circle.Some embodiments of the invention also include at least two lightssources, each light source corresponding to one of the imaging devices,where the light sources are mounted above the at least two imagingdevices. In these embodiments, the two light sources may be mountedapproximately 18 inches above the imaging devices.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description given below, serve to explain the invention.

FIG. 1 is schematic diagram of a multi-camera arrangement for assessmentof cardiovascular functions consistent with embodiments of theinvention;

FIG. 2 is an alternate embodiment of the multi-camera arrangement ofFIG. 1;

FIG. 3 is another alternate embodiment of the multi-camera arrangementof FIG. 1;

FIG. 4 is still another alternate embodiment of the multi-cameraarrangement of FIG. 1;

FIG. 5 is a schematic diagram of camera triggering and data flow fromthe multi-camera arrangements consistent with the embodiments in FIGS. 1through 4;

FIG. 6 is an exemplary flow diagram of a process pipeline consistentwith schematic diagrams of FIGS. 1 through 5;

FIG. 7 is an exemplary flow diagram illustrating extraction of Cardiacinformation from the multi-camera arrangement of FIG. 1;

FIGS. 8A and 8B are graphs containing an exemplary periodogram fordetermining pulse rates from an automatically selected ncPPG componentand an equivalent periodogram from a fingertip PPG time series; and

FIG. 9 is a graph containing exemplary time series data from ECG, PPG,and recovered ncPPG components.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the sequence of operations as disclosedherein, including, for example, specific dimensions, orientations,locations, and shapes of various illustrated components, will bedetermined in part by the particular intended application and useenvironment. Certain features of the illustrated embodiments have beenenlarged or distorted relative to others to facilitate visualization andclear understanding. In particular, thin features may be thickened, forexample, for clarity or illustration.

DETAILED DESCRIPTION OF THE INVENTION

Photoplethysmography (PPG) relies on characteristic changes in theoptical absorption of tissue due to pulsatile (arterial) blood flow inperipheral vasculature. Sensors for observing the photoplethysmographiceffect have traditionally required contact with the skin surface. Recentadvances in non-contact imaging PPG have demonstrated that measures ofcardiopulmonary system state, such as pulse rate, pulse ratevariability, and respiration rate, can be obtained from a participant byimaging their face under relatively motionless conditions. A criticallimitation in this method that must be resolved is the inability torecover these measures under conditions of head motion artifact.

Embodiments of the invention address the need in the art by utilizingmultiple synchronized, visible spectrum imagers positioned in an arraycentered on an imaged participant. Increased measurement error duringmotion artifact is mitigated by the embodiments of the invention by theincreased dimensionality of the imager channel space with multipleimagers in the array prior to applying a blind source separation. Incontrast to single-imager results, the multi-imager channel spaceresults in an absolute error in the recovered pulse rate measurementthat is comparable with pulse rate measured via fingertip reflectancephotoplethysmography. These results demonstrate that non-contact,imaging photoplethysmography can be accurate in the presence of headmotion artifact when utilizing embodiments of the invention implementinga multi-imager array to increase the dimensionality of the decomposedchannel space.

Embodiments of the invention replace traditional contact, cardiacmonitoring with a synchronized array of digital video cameras collectingvideo data of an individual's face and neck. Turning now to thedrawings, wherein like numbers denote like parts throughout the severalviews, FIG. 1 illustrates an exemplary embodiment including an array ofnine synchronized digital video cameras 10 a-i in a semi-circulararrangement around an individual 12 capturing simultaneous views 14 a-iin front of an arbitrary background 16. Cardiac information results froman analysis and signal processing of small variations in reflected colorinformation from an individual's 12 skin. This process is referred to asnon-contact photoplethsymography (ncPPG). ncPPG relies on the samephysical phenomena as traditional, contact PPG. The variations inreflected color intensity result from the fluctuating absorption andreflection of the blood volume pulsing through the peripheralvasculature of the skin throughout the cardiac cycle. In ncPPG, thesevariations are collected at a distance 18 from an imager 10 a-i, insteadof at the skin's surface with a sensor. Utilizing an array of imagers,such as cameras 10 a-i, provides simultaneous views of the subject frommultiple angles with continuous coverage of the face, even duringmovement. This allows the fluctuating PPG component to be continuouslyrecorded, despite the orientation or movement of the individual 12 inframe. The improved and continuous views of the individual 12 providemore and higher quality inputs into a signal processing pipeline (seeFIG. 6) (specifically a blind source separation component) allowing fora robust collection of cardiac information during movement. Embodimentsof the invention show great improvements from single imager setupsdocumented elsewhere which are highly susceptible to motion artifacts.

In the exemplary embodiment illustrated in FIG. 1, the nine imager 10a-i semi-circular array consists of Basler Scout acA640-120gc GigEstandard color progressive scan cameras with global shutter, capable ofrendering 12-bit RGB pixel depth images at VGA resolution (640×480pixels) at 120 frames per second (fps), though other comparable camerasfrom other manufacturers may also be utilized. Each imager is equippedwith a 16 [mm] fixed focal length lens to maximize an area in each frameoccupied by the individual's 12 face. In this illustrated embodiment,each camera 10 a-i is supported by a tripod and positioned to align thecamera 10 a-i with the level of the individual's 12 face. Otherembodiments may utilize other methods of placing cameras around theindividual 12 by, for example, fixing the cameras to the surroundingenvironment or by suspending the cameras. The cameras 10 a-i are spacedto occupy approximately a 160° semi-circle centered on the individual 12with about 20° between each camera 10 a-i. The total range of the array,±80° from normal was chosen based on a maximum normal head rotationrange of an individual 12, though other ranges, separations, and numberof cameras may also be used. For example, as illustrated in FIG. 2, athree camera set up covering 40 degrees is illustrated. Much of thequality input may be collected from the center cameras, especially whenmovement of the individual 12 may be limited. In FIG. 3, a five camerasetup covering 120 degrees is illustrated. In this embodiment, thecameras are spaced 30 degrees apart. FIG. 4, illustrates a four camerasetup also coving 120 degrees. In this illustrated embodiment, thecameras are spaced 40 degrees apart. One of skill in the art willrealize that number of cameras and spacing thereof can have any numberof configurations and that such configurations will likely be dictatedby the environment of the individual 12, whose cardiac information isbeing monitored. Returning to FIG. 1, the cameras 10 a-i are positionedat a distance 18 of 68 inches away from the individual 12. Again, thisdistance is somewhat arbitrary and distances used in other embodimentsmay be dictated by the types of cameras being used as well as theoverall environment in which the cameras are positioned.

Lighting (not shown) for the camera 10 a-i array is provided by 10 SoLuxblack-back, full color spectrum (5000K) bulbs with a 36° beam spread.These bulbs simulate natural daylight spectrum, with nearly constantoutput power from 480 to 1080 [nm]. This additional lighting may beincluded or omitted in other embodiments, again depending on thelighting of the environment in which the cameras are positioned. Theseparticular light sources are mounted approximately 18 inches above eightof the imagers 10 b-i (the exception being the middle imager 10 a in thearray) and perpendicular to the normal vector of the imager. Additionallight sources 20 may be positioned beyond the most extreme cameras 10 h,10 i at the −100° and +100° positions of the semi-circle to ensure moreeven lighting. Again this additional lighting may be included or omittedin other embodiments, again depending on the lighting of the environmentin which the cameras are positioned.

Turning now to FIG. 5, the camera array 10 a-i is synchronized with aNational Instruments (NI) data acquisition card (DAQ) 22, modelPCIe-6323, mounted in a workstation-class PC. Other embodiments of theinvention may use other DAQ hardware or other sychnonizing schemes. TheDAQ outputs a hardwire-timed pulse 24 train at 120 Hz and 90% duty cycleto the input controls of the nine imagers 10 a-i. This ensuressynchronous exposure and capture of the cameras across the array. Whilepulse train 24 is used in this illustrated embodiment, other embodimentsof the invention may use other triggers to synchronize the outputs ofthe cameras 10 a-i.

The imager data is collected using three NI quad port GigE videoacquisition boards, model PCIe-8235, mounted in three separateworkstation-class PCs 26 a-c. Each PC 26 a-c and three correspondingcameras form a pod whose raw, BG BAYER format data is recorded as abinary stream to a PCIe-mounted solid state hard drive. The podconfiguration and solid state hard drives are used to support the dataacquisition rate necessary for the simultaneous acquisition of imagerarray. While the illustrated embodiment utilizes the PC and camera podconfiguration, other hardware configurations may also be used utilizingmore or fewer computer or other electronic programmable devices, e.g.,multi-user or single-user computers, desktop computers, portablecomputers and devices, handheld devices, network devices, mobile phones,etc. The number and of devices may be dictated based on the number ofcameras capturing images or processing capability of the device.

The stored raw, BG BAYER format binary data is recalled, though in otherembodiments, instead of storing data for later recall, the processing ofthe data may be accomplished in real time. The raw data is demosaiced toform an RGB image, such as 28 a and 28 d as seen in FIG. 6. Mean/averageRGB traces such as 30 a, 30 d are then calculated and recorded for theentire image frame. These mean RGB signals serve as the raw signal toextract the desired measures of cardiac activity. Other embodiments maychoose different regions of interest from which to extract mean RGBtraces. These embodiments may define the region of interest through facetracking, skin pixel detection, or other means, for example.

The mean RGB signals may be processed following contemporary processingroutines, such as by Poh, McDuff, and Picard (2011). Notable exceptionsto the contemporary signal processing routine are the use of 27 channelsfor ICA (9 cameras×RGB) vs. 3 channels (1 camera×RGB) and an automaticidentification of the PPG component. Turning to flowchart 32 in FIG. 7,this process begins at block 34 and includes the following steps:

A first signal of the 27 mean RGB signals is selected in block 36. Thesignal is filtered with a band-pass filter between about 0.3 Hz andabout 10 Hz in block 38 to eliminate any signal components that areoutside a range of a typical cardiac signal. The filtered signal is thenresampled from about 120 Hz to about 1,200 Hz using a splineinterpolation to refined temporal resolution of the signal in block 40.Other embodiments may resample the signals using other frequencies andinterpolations to improve temporal resolution. If there are additionalsignals (“Yes” branch of decision block 42), then the next signal isselected in block 44, and the filtering 38 and resampling 40 is repeatedfor this signal. If there are no additional signals, (“No” branch ofdecision block 42), then the filtered, resampled signals are input intoan extended infomax, independent component analysis (ICA) algorithm inblock 46. ICA is a type of blind source separation, which isolatesindependent components present in mixed data sources. This isaccomplished through iterative maximization of non-gaussanity. From thealgorithm, a number of underlying, independent components are output;one of which represents the PPG signal of interest while the otherscontain noise sources like movement. Other embodiments may use othertypes of analyses to isolate signal components.

After ICA, all independent components were bandpass filtered in block 48with a zero-phase elliptical band pass filter between 0.3-6 (Hz) beforeupsampling the signal to 1200 (Hz) using cubic spline interpolation. Asbefore, other filter and sampling of the signals may be used. Thecomponent most likely to contain PPG information is automatically chosenbased on an estimate of signal-to-noise ratio (SNR) in the expectedfrequency range of the ncPPG component signal. In one particularembodiment, the method used a periodogram approach (with a Hammingwindow) to estimate the PSD of the components. Looking specifically atthe frequencies and corresponding power densities from 0.75-4 (Hz) inblock 50 (the likely pulse rate range corresponding to 45-240 (bpm), thefrequency with maximum power density is located and band power for thatdominant frequency is calculated over a range of ±0.123 (Hz).

The cardiac information from the PPG component is then extracted inblock 52. In this illustrated embodiment, the dominant frequency bandpower estimate is transformed to relative power by dividing by thecomponent signal power outside the peak frequency range but within0.75-4 (Hz). The pulse rate component is chosen as the independentcomponent with the highest relative power, as a measure of SNR, and thenvisually confirmed as the correct component. The resulting component isthen segmented into 5, non-overlapping, 1 (min) windows (discarding the15 (s) pre- and post-buffers). Mean pulse rate for each window wascalculated using a periodogram method (with a Hamming window) by findingthe frequency with the highest power density in the probable pulse raterange (0.75-4 (Hz) and converting this frequency to [bpm] (60·f_(max)).An example of this method for determining pulse rate from theautomatically selected ncPPG component is shown in FIG. 8A. As a groundtruth comparison, the same method for calculating probable pulse ratewas applied to a fingertip PPG data source, shown in FIG. 8B. Whilepower density was used by this illustrated embodiment to extract pulsedata, other embodiments may use other extraction methods. The processends at block 54.

An example set of time series data from ECG, PPG, and the recoveredncPPG component is shown in FIG. 9. Indicator lines 60 (Blue), 62(Green), and 64 (Red) mark the beginning of the systolic phase in theECG, PPG, and ncPPG time series, respectively. From this data segment,it is clear that the ncPPG source component was accurately recoveredfrom the 9-imager channel space decomposition. Relative timing ofsystolic onset between ECG, PPG, and ncPPG is consistent with realisticphysiological constraints such as pulse wave arrival time being delayedin peripheral vasculature (in the fingertip) as compared to the ncPPGcomponent (facial vasculature).

The results demonstrate the value of a multi-imager acquisition approachfor reducing pulse rate recovery error with ncPPG. While thesingle-imager approach iPPG may be sufficient for situations in whichhead movement artifact is prevented, even a quasi-stationary participantwill produce noticeably larger errors in pulse rate measurement. Bycontrast, the multi-imager array is successful at reducing pulse raterecovery error to approximately 2 (bpm), even during head motionartifact. This magnitude of error is comparable to the lowest error,under the most favorable conditions, that can be obtained with a singleimager and is most promising for the development and transition of ncPPGsystems to everyday, natural environments where non-contactcardiopulmonary assessment is either desirable or necessary.

While the present invention has been illustrated by a description of oneor more embodiments thereof and while these embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus andmethod, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thescope of the general inventive concept.

What is claimed is:
 1. A method for non-contact cardiac assessment of asubject, the method comprising: capturing images of the subject from atleast two synchronized independent imaging devices spaced equidistantfrom the subject and positioned such that each imaging device capturesan image of the subject different from other imaging devices; extractingspectral components of the captured images; analyzing the extractedspectral components; identifying a signal in the extracted spectralcomponents corresponding to cardiac information; and extracting thesignal in the extracted spectral components corresponding to the cardiacinformation.
 2. The method of claim 1, wherein capturing the images ofthe subject from at least two synchronized independent imaging devicescomprises: capturing images of a head and neck of the subject from theat least two synchronized independent imaging devices.
 3. The method ofclaim 1, wherein extracting spectral components of the captured imagescomprises: extracting Red, Green, and Blue components of the capturedimages.
 4. The method of claim 1, wherein analyzing the extractedspectral components comprises: performing an independent componentanalysis on the extracted spectral components.
 5. The method of claim 1,wherein identifying a signal in the extracted spectral componentscorresponding to cardiac information comprises: bandpass filtering theextracted spectral components between about 0.3 Hz and about 6 Hz;upsampling the extracted spectral components about 1200 Hz using cubicspline interpolation; and choosing a signal from the extracted spectralcomponents based on an estimate of signal-to-noise ratio (SNR) in anexpected frequency range of a ncPPG component signal.
 6. The method ofclaim 5, wherein the expected frequency range of an ncPPG componentsignal is about 0.75 Hz to about 4 Hz.
 7. The method of claim 1, whereinextracting the signal in the extracted spectral components correspondingto the cardiac information comprises: transforming band power estimateto relative power by dividing by component signal power outside a peakfrequency range but within about 0.75 Hz top about 4 Hz; and extractingan independent component with a highest relative power.
 8. The method ofclaim 7, further comprising: segmenting the extracted component intofive non-overlapping windows; and finding a frequency with a highestpower density in a probable pulse rate range and converting thisfrequency to beats per minute by 60·f_(max), wherein f_(max) is thefrequency with the highest power density.
 9. A system for non-contactcardiac assessment of a subject, the system comprising: at least twoimaging devices configured to individually capture images of thesubject, the imaging devices being spaced equidistant from the subjectand positioned such that each imaging device captures an image of thesubject different from other imaging devices; a data acquisition card inelectrical communication with the at least two imaging devices; and atleast one computer in electrical communication with the at least twoimaging devices and configured to: receive captured images from the atleast two imaging devices; extract spectral components of the capturedimages; analyze the extracted spectral components; identify a signal inthe extracted spectral components corresponding to cardiac information;and extract the signal in the extracted spectral componentscorresponding to the cardiac information, wherein trigger signalsgenerated by the data acquisition card cause images to be capturedsimultaneously by the at least two imaging devices and sent to thecomputer.
 10. The system of claim 9, wherein the subject is positionedat a distance of about 68 inches from the at least two imaging devices.11. The system of claim 9, wherein the system comprises three imagingdevices about 20 degrees apart, spanning about 40 degrees of visibilityof the subject.
 12. The system of claim 9, wherein the system comprisesfive imaging devices about 30 degrees apart, spanning about 120 degreesof visibility of the subject.
 13. The system of claim 9, wherein thesystem comprises nine imaging devices about 20 degrees apart, spanningabout 160 degrees of visibility of the subject.
 14. The system of claim13, further comprising: at least two lights positioned beyond imagingdevices positioned at about ±80 degrees.
 15. The system of claim 9,wherein the system comprises four imaging devices about 40 degreesapart, spanning about 120 degrees of visibility of the subject.
 16. Thesystem of claim 9, wherein the at least two imaging devices arepositioned at a height to captures images of a head and neck of thesubject.
 17. The system of claim 9, wherein the at least two imagingdevices positioned along a semi-circle.
 18. The system of claim 9,further comprising: at least two lights sources, each light sourcecorresponding to one of the at least two imaging device, wherein thelight sources are mounted above the at least two imaging devices. 19.The system of claim 18, wherein the at least two light sources aremounted approximately 18 inches above the at least two imaging devices.20. The system of claim 9, wherein the at least two imaging devices arevisible spectrum imaging devices.